1. Field of the Invention
The present invention relates to the art of printers, and more particularly, to a sheet supply and transfer device for printers for transferring sheets of a rectangular contour as transferred from a stack thereof one by one through a printing portion of the printer.
2. Description of the Prior Art
FIG. 1 is a side view showing somewhat diagrammatically an example of a rotary type stencil printing apparatus conventionally known and actually used. In this figure, a generally cylindrical printing drum 10 has perforated construction in its principal portion of its cylindrical circumference, except its annular edge portions at its axially opposite ends and a transverse bar portion 12 bridged therebetween, as well known in the art. A perforated stencil sheet is mounted at its leading edge to a clamp means (not shown) provided on the transverse bar portion 12 and is wrapped around the cylindrical circumference of the printing drum toward its trailing edge. The printing drum is driven by a driving means not shown in the figure to rotate around a central axis thereof in the counter-clockwise direction in the figure, with ink being supplied from its inside by an ink supply means not shown but well known in this art. A paper drum 14 is of the same diameter as the printing drum 10, formed with transverse groove 16 at a portion thereof for avoiding interference with the transverse bar portion 12 of the printing drum 10. The paper drum 14 is driven to rotate around its central axis in the clockwise direction in the figure in synchronization with the counter-clockwise rotation of the printing drum 10 via a linkage mechanism not shown in the figure. A printing portion is established by the printing drum 10 and the paper drum 14, to which, in synchronization with the rotation of the printing drum 10 and the paper drum 14, an uppermost sheet of a stack 20 of regular sheets supported on a sheet supply tray 18 is fed one by one by a sheet feed out roller 22, so that each sheet is transferred through a nip portion between the printing drum 10 and the paper drum 14, so as to be applied with an image of ink on its upper surface according to the perforations of the stencil sheet wrapped around the printing drum 10 under a back pressing by the paper drum 14, the sheet being further guided through a nip portion of a first pair of pinch rollers 24 and the paper drum 14 rightward in the figure, and further being transferred through a nip portion between a second pair of pinch rollers 26 and a discharge roller 28 toward a sheet discharge tray 30. The first and second pairs of pinch rollers 24 and 26 are adapted to be driven by an output shaft 32 of a pinch roller drive motor (not shown) via an endless belt 34.
The pinch rollers 24 and 26 contact the upper surface of the sheet applied with the image of ink, i.e. a print. Therefore, these pinch rollers should contact the sheet only in a pair of narrowly limited regions extending along the opposite side edges of the sheet. Therefore, the pair of pinch rollers adapted to contact the sheet along those narrowly limited side edge portions thereof need to be adjusted of their positions so as to meet with the width of the paper sheets for printing which changes according to a kind of the paper sheets such as, for example, sizes A4, B4, etc.
On the other hand, the sheet supply tray 18 is provided with a pair of side fences 36 for properly aligning the opposite side edges of the stack of sheets 20 charged thereon. Therefore, in some printers of this type advanced in automation are so constructed that the spacing between the pair of side fences 36 adjustable of the positions thereof according to the width of printing sheets is detected by an appropriate sensor, and the pair of pinch rollers are automatically positioned based upon the detected magnitude of the spacing.
Such a construction that the spacing between the side fences of the sheet supply tray is detected by a sensor and the positions of a pair of pinch rollers are automatically set up operates effectively when sheets are correctly stacked on the tray and the side fences are properly set up against the stack of sheets. However, when such a premise is deformed, the deformation directly affects the positioning of the pinch rollers, rendering the positioning of the pinch rollers relative to the width of the actual sheets not to be proper.
In automatically setting up the pair of pinch rollers by detecting a position of the side fences, it is generally conventional to link the movement of the pair of side fences with one another by a linking mechanism such as a rack-and-pinion, so that the pair of side fences move symmetrically relative to one another with respect to its middle point in their approaching or departing movement for decreasing or increasing the spacing therebetween, while the pair of pinch rollers are also moved symmetrically to one another relative to the center line in accordance with corresponding movement of the side fences. According to such a conventional system, there can occur such conditions as shown in FIGS. 2A, 2B and 2C. In more detail, in the case of FIG. 2A, although the stack 20 of sheets is properly formed, one or both of the pair of side fences (both in the shown example) are not properly contacted to the stack of sheets, as being apart therefrom. In such a condition, when the pair of pinch rollers 26 are set up according to the distance between the side fences, a condition such as shown in FIG. 2a occurs, wherein the opposite side edges of the sheet are not sufficiently pressed by the pinch rollers or not pressed at all as in the shown example.
In an example shown in FIG. 2B, each sheets forming the stack 20 are randomly displaced relative to one another in their width. Such a condition can often occur in the second and subsequent printing of a multi-color printing. In this case, although the pair of side fences are sufficiently contacted to the opposite side edges of the stack of printing sheets as a whole, each sheet is not properly aligned as shifted on one side or the other, so that on either side the pinch roller can not properly press the sheet as shown in FIG. 2b.
FIG. 2C shows a case that a small number of sheets are stacked to form a relatively thin stack of sheets which is readily bendable when the pair of side fences are set up too close to one another. In this case, as shown in FIG. 2c, the pair of pinch rollers are biased to the inside of the sheets from proper positions which might cause the pinch roller to roll on a printed image.